Mead A. Allison

Large-river delta-front estuaries as natural recorders of global environmental change

Large-river delta-front estuaries (LDE) are important interfaces between continents and the oceans for material fluxes that have a global impact on marine biogeochemistry. In this article, we propose that more emphasis should be placed on LDE in future global climate change research. We will use some of the most anthropogenically altered LDE systems in the world, the Mississippi/Atchafalaya River and the Chinese rivers that enter the Yellow Sea (e.g., Huanghe and Changjiang) as case-studies, to posit that these systems are both “drivers” and “recorders” of natural and anthropogenic environmental change. Specifically, the processes in the LDE can influence (“drive”) the flux of particulate and dissolved materials from the continents to the global ocean that can have profound impact on issues such as coastal eutrophication and the development of hypoxic zones. LDE also record in their rapidly accumulating subaerial and subaqueous deltaic sediment deposits environmental changes such as continental-scale trends in climate and land-use in watersheds, frequency and magnitude of cyclonic storms, and sea-level change. The processes that control the transport and transformation of carbon in the active LDE and in the deltaic sediment deposit are also essential to our understanding of carbon sequestration and exchange with the world ocean—an important objective in global change research. U.S. efforts in global change science including the vital role of deltaic systems are emphasized in the North American Carbon Plan (www.carboncyclescience.gov).

The science of hypoxia in the Northern Gulf of Mexico: a review.

The Mississippi River is one of the worlds 10 largest rivers, with average freshwater discharge into the northern Gulf of Mexico (GOM) of 380km(3) year(-1). In the northern GOM, anthropogenic nitrogen is primarily derived from agricultural fertilizer and delivered via the Mississippi River. The general consensus is that hypoxia in the northern Gulf of Mexico is caused primarily by algal production stimulated by excess nitrogen delivered from the Mississippi-Atchafalaya River Basin and seasonal vertical stratification of incoming stream flow and Gulf waters, which restricts replenishment of oxygen from the atmosphere. In this paper, we review the controversial aspects of the largely nutrient-centric view of the hypoxic region, and introduce the role of non-riverine organic matter inputs as other oxygen-consuming mechanisms. Similarly, we discuss non-nutrient physically-controlled impacts of freshwater stratification as an alternative mechanism for controlling in part, the seasonality of hypoxia. We then explore why hypoxia in this dynamic river-dominated margin (RiOMar) is not comparable to many of the other traditional estuarine systems (e.g., Chesapeake Bay, Baltic Sea, and Long Island Sound). The presence of mobile muds and the proximity of the Mississippi Canyon are discussed as possible reasons for the amelioration of hypoxia (e.g., healthy fisheries) in this region. The most recent prediction of hypoxia area for 2009, using the current nutrient-centric models, failed due to the limited scope of these simple models and the complexity of this system. Predictive models should not be the main driver for management decisions. We postulate that a better management plan for this region can only be reached through a more comprehensive understanding of this RiOMar system-not just more information on river fluxes (e.g., nutrients) and coastal hypoxia monitoring programs.

Stratigraphic evolution of the late Holocene Ganges–Brahmaputra lower delta plain

Sediment cores from the Ganges–Brahmaputra delta in Bangladesh were examined for sedimentological character, clay mineralogy, elemental trends (C, N, S), and 14C geochronology to develop a model for the sedimentary sequence resulting from lower delta plain progradation in the late Holocene. A widespread facies succession from Muddy Sand to Interbedded Mud records progradation of shoal–island complexes and the transition from subtidal to intertidal conditions. Mangrove-vegetated islands and peninsulas represent the final phase of progradation; a Mottled Mud that is deposited by penetration of turbid coastal water into the mangroves during high water events. Organic matter preservation is generally low (<1% TOC) in most of these well-drained deposits that are characterized by a permeable, silt-dominated granulometry. Clay mineralogy in the cores records the relative influence of smectite and kaolinite-rich Ganges sediments and illite and chlorite-rich Brahmaputra material. The lower delta plain west of the modern river mouths was deposited as a Ganges-dominated delta in three phases since 5000 cal years BP, with Brahmaputra influence confined to the Meghna estuary area and to the supratidal section of western delta deposits. Evolution of the lower delta plain in the late Holocene was influenced by regional subsidence patterns in the tectonically active Bengal Basin, which controlled distributary channel avulsion and migration, and the creation of accommodation space.

Origin of Amazon mudbanks along the northeastern coast of South America

Abstract Seismic profiles, sediment cores, and water column measurements were collected along the northeastern coast of Brazil to examine the origin of mudbanks in the Amazon coastal mud belt. These 10–60-km-long, shore-attached features previously had been observed to migrate along the 1200 km coast of the Guianas in response to wave forcing. CHIRP (3.5 kHz) seismic profiles of the shoreface and inner shelf located two mudbanks updrift of the previous eastern limit in French Guiana. 210 Pb geochronology shows that these two banks are migrating to the northwest over a relict mud surface in 5–20 m water depth. The mudbanks are 3–4 m thick and are translating over a modern shoreface mud wedge deposited by previous mudbank passage in

Bedform transport rates for the lowermost Mississippi River

[1] New methods of data collection and processing are developed to provide quantitative, reach-scale measurements of bedform transport mass within the tidally influenced Mississippi River. A multibeam swath profiler was used to collect daily bathymetry over a range of water discharges, and bed elevation changes induced by dune migration are measured. These values are coupled with bulk physical properties of the bed sediment to constrain mass flux, and annual bedform transport is estimated at 2.2 x 10 6 metric tons (MT). The total annual sand flux from the Mississippi River, calculated by combining measured bedform transport rates and suspended sediment flux, is estimated to be 20 x 10 6 MT. Survey data also provide information about the spatial distribution of dunes across the channel bottom. Straight reach segments are commonly mantled by dunes for the entire cross section, while bends are typically areas of focused scour devoid of bedforms. Presumably, any sediments associated with migrating dunes are propelled into suspension within bends before redepositing in the subsequent straight reach. Movement via suspension is therefore an important component for the downriver transport of bed materials in the lower Mississippi River.

Importance of flood-plain sedimentation for river sediment budgets and terrigenous input to the oceans : Insights from the Brahmaputra-Jamuna River

Recent authors have suggested that a significant proportion of the worldwide terrigenous sediment budget is trapped landward of the river-ocean boundary in high-load, tectonically active basins. To test this idea, modern flood-plain sediment accumulation rates were determined along a 110 km reach of the Brahmaputra (locally Jamuna) River using 137Cs geochronology of sediment cores and geographic information system (GIS) extrapolation to adjacent areas. 137Cs accumulation rates decrease exponentially away from the channel, from >4 cm/yr on the natural levees to <1 cm/yr within a few tens of kilometers basinward. Important controls on sedimentation in this area include proximity to distributary channels, local topography, and interannual variability of the flood pulse. Model results indicate that an average of 23 m.t./yr of sediment are sequestered in this section of the flood plain. Extrapolated basinwide, as much as 39%–71% of the river sediment budget may be trapped landward of the Ganges-Brahmaputra mouth.

Sediment deposition, accumulation, and seabed dynamics in an energetic fine-grained coastal environment

Sedimentary processes on the continental shelf and shoreline northwest of the Amazon River mouth were investigated as part of A Multidisciplinary Amazon Shelf SEDiment Study (AmasSeds) during four field expeditions between 1989 and 1991. Periodic deposition and resuspension of seabed layers as much as a meter thick dominate sedimentary processes for most of the inner shelf and for the shoreface and foreshore north of Cabo Cassipore. Strata forming as a result of this process consist of decimeter-thick mud beds separated by hiatal (scour) surfaces. The volume of sediment resuspended seasonally from the inner shelf surface layer (SL) is of the same order of magnitude as the annual input from the river, indicating that resuspension is an important control on suspended-sediment distributions in shelf waters. Most resuspension from the SL occurs during February–May (the period of maximum wind stress), which is also the time of rapid deposition on the mudflats, suggesting that sediment resuspended from the SL could contribute to shoreface and foreshore accretion for the northern portion of the study area. In addition, some of the sediment resuspended from the SL is transported seaward periodically in the form of near-bottom fluid-mud flows. This results in non-steady-state input of certain particle-reactive trace metals, which is reflected in the occurrence of quasi-cyclic210Ph profiles in the foreset region of the subaqueous delta. As determined using228Ra/226Ra geochronology, sediment accumulation rates in this region are 10–60 cm y−1. Farther seaward, in the bottomset region, accumulation rates decrease and there is increased evidence of biological activity preserved in sedimentary structures. However, episodic (but reduced) sediment input from fluid-mud flows also extends to this region, affecting the fauna and fine-scale stratigraphy.

The geological record preserved by Amazon shelf sedimentation

Abstract A recent study of the subaqueous delta and coastal plain near the mouth of the Amazon River provides insight to the geological record created there and elsewhere. A compound clinoform structure is forming across the Amazon shelf. The uppermost portion is the shoreline, whose aggradation brings the modern sedimentary deposit to sea level and produces a deposit 5–10 m thick. It contains sediments accumulating primarily in shallow subtidal areas, intertidal mudflats and mangrove forests, and progradation occurs by overlapping of northward-extending mudcapes. Through these processes, the coastal plain has been widened by 10–100 km during the Holocene. The shoreline deposits are prograding across topset strata of the modern subaqueous delta, which is the lowermost and dominant portion of the compound clinoform structure. The subaqueous delta extends to a water depth of 70 m, with a depositional break between topset and foreset strata at 30–40 m. Advective sediment input to the foreset region causes high accumulation rates, which control the geometry and progradation of the clinoform structure. On time scales of 102−103 y, physical processes (e.g. waves, currents) have changed and the upper portions of the coastal plain and subaqueous delta have been eroded. One expression of this is a widespread unconformity recorded within late Holocene strata on the inner shelf. Over longer time scales (103−104 y) sea-level changes have led to more extensive erosion and only the lower 20 m of the compound clinoform structure is preserved, overlain by a transgressive sand layer. On other continental margins with different regional characteristics (e.g. more rapid subsidence) larger fractions of the clinoform structures could be preserved.

Seasonal sediment storage on mudflats adjacent to the Amazon River

Abstract 210Pb and 234Th activity profiles in sediment cores from underconsolidated mudflats 300 km downdrift of the Amazon river mouth record an ephemeral surface layer of fine-grained sediment up to 1.5 m thick. This layer contains about l.5 × 108 tons of Amazon sediment deposited rapidly (~1 cm/d) from a fluid-mud suspension (10–400 g/l) during the months between January and June. Virtually the entire layer is remobilized in July–December and the sediment is advected alongshore to the northwest. Seasonal variations in trade-wind strength and in supply of Amazon shelf sediment are thought to control emplacement, and removal of this ephemeral deposit. Solitary surface gravity waves characteristic of this setting generate a net landward sediment flux, which, with shore-normal tidal currents, controls spatial geometry of the surface layer. The resultant lens-shaped deposit dissipates incident wave energy and provides a substrate above mean high water for mangrove colonization and irregular shoreline progradation of meters per year. Macroscale (sand/silt laminations) and microscale (plasmic fabric) sedimentary structures in the ephemeral layer record diverse temporal variations (e.g., tidal and wave-induced) in bottom shear stress and sediment supply. Ephemeral deposition of 108 tons is inferred to be common in coastal areas associated with large and energetic river dispersal systems.

Rates and mechanisms of shoreface progradation and retreat downdrift of the Amazon river mouth

Field surveys of the 350-km shoreline adjacent to the Amazon river mouth reveal three distinct types: erosional mud, accretionary sand, and accretionary mud. Formation of these zones is controlled by the delivery of Amazon suspended sediment, mediated by the hydrodynamic regime. Erosional mud shorelines extend from Rio Araguari (near the Amazon river mouth) northwestward 280 km to 3.5 °N (near Cabo Cassipore). Shoreface (<5 m water depth) retreat annually yields 1–4 × 106 tons of fine-grained sediment deposited during an earlier phase of shoreface progradation. Sandbodies up to 5 m thick overlie erosional mud shorefaces for 10–30 km downdrift of small rivers. The sand is supplied from these rivers and released by shoreline retreat of the river-mouth areas. Amazon River sand is restricted in the coastal zone from its mouth north to the Cabo Norte shoal area, where it mixes with sand carried by Rio Araguari from the Guiana Highlands. North of 3.5 °N, mud aggradation and progradation is taking place on underconsolidated, low-gradient tidal flats backed by mangrove swamps. 210Pb and 14C geochronology of vibracores from the mudflats indicate that sediment accumulation is rapid (0.24–2.0 cm/yr) landward of the 2-m isobath, supplied from a thick (50–150 cm) seasonal surface layer. Shoreface progradation is episodic, separated by decadal hiatuses. Fine-grained suspended-sediment flux from the Amazon and minor amounts of sand and mud from the local rivers supply sediment to the mudflats. Shore-normal tidal currents and solitary waves rework the surface mud layer, preferentially transporting available sand landward into the mangrove fringe, and producing very fine-grained accumulation on the tidal flat (10–12 φ mean grain size). Lateral accretion of features 10–100 km long, termed mudcapes, produces tens of kilometers of seaward coastal-plain addition along the northern coast. Similar features are identified downdrift along the Guianas coast as far as the Orinoco River (1600 km). The northern Amapa shoreface deposits are a locus of modern sediment accumulation, which progrades over subaqueous deltaic strata.